The present invention is in the field of hydraulic boosters; more particularly, the invention relates to a pressure limiter for use with hydraulic boosters.
The use of pressure limiting devices in hydraulic boosters is known in the art. As shown by U.S. Pat. Nos. 3,728,942; 3,845,693 and 3,796,134, means are provided to limit the pressure below a predetermined value.
Hydraulic boosters are used to increase the pressure in the system. This increased pressure can be used, as in the case of a power braking system, to assist in braking. There are means to limit the hydraulic booster to operate below a predetermined pressure. In hydraulic boosters of the type which use demand or gain valves to restrict a fluid passage or gap, the pressure between the gap and the pumping means increases. Stops have been used to limit the amount of restriction in the gap of the gain valve and thereby limit the pressure. There must be sufficient flow through the gap for sufficient fluid for the power steering gear.
U.S. Pat. No. 3,728,942 is an example of a gain valve restriction which is limited by the action of a stop means or stem on an adjusting stud intercepting a washer on the gain valve spool or piston so that the restriction remains opened. A predetermined maximum pressure can be set by adjusting the distance between the stem and the washer.
U.S. Pat. No. 3,845,693 has a particular variation of this type of approach of limiting pressure but also assuring that the pressure limiting device will not interfere with brake pedal operation. However, there is no control over the booster pressure acting in the hydraulic booster other than to cause a predetermined limit based on the given value of the restriction opening used to create the pressure.
U.S. Pat. No. 3,796,134 has a pressure limiting means where the pressure between the pumping means and the hydraulic booster is closed above a given pressure.
Generally, prior art hydraulic booster pressures are limited based on mechanical predetermined spatial relationships within the valve to limit the pressure of a known fluid passing through the gain or demand valve. The dimensional tolerances in the prior art hydraulic boosters must be very carefully controlled. If the gap is too small, there is insufficient fluid to the power steering gear and if the gap is too large, the desired booster pressure is limited below the maximum desirable value. Prior art pressure limiters cannot compensate for pressure changes which might result through the use of fluids having different properties or for the effect of temperature changes on fluid properties.